The third generation of biofuels is both promising and different: it is based on simple microscopic organisms that live in water and grow hydroponically. These micro-algae do not need soil and land, and because many of them thrive in water that is salty, brackish or just plain dirty – wastewater or agricultural run-off, for example – they need not compete for scarce fresh water resources either. Also important, they are far more productive than terrestrial fuel crops.

Given plenty of sunlight, these organisms can photosynthesise enough organic matter, from carbon dioxide (CO₂) and organic nutrients present in the water they are suspended in, to double their mass several times a day. Depending on the species, up to half their mass is made up of lipids – natural oils. These can be extracted and used as straight algal ‘crude’, or refined to higher-grade hydrocarbon products ranging from biodiesel to biojet fuel for aircraft. Strains of algae that produce more carbohydrate than oil can be fermented to make bioethanol and biobutanol.

Algae biofuels contain no sulphur, are non-toxic and are biodegradable. A number of strains produce fuel with energy densities comparable to those of conventional (fossil) fuels. They are made from a renewable resource that is carbon neutral: the emissions that result from burning the fuel are balanced by the absorption of CO₂ by the growing organisms.

Small wonder that these miracle organisms are the subject of intense study. Algae are familiar to the general public as pond scum and to oceanographers as the algal blooms that blossom over huge areas of ocean at certain times of year. The abundance of wild algae and the lipid nature of many of them have engendered high optimism about their potential as fossil fuel substitutes. But exploiting the potential of a technology that currently exists only at laboratory and pilot scale could prove a long and expensive undertaking. For a start, isolating a couple of score that might make a viable basis for fuel production from the 30,000 or so existing algal strains represents a formidable challenge. Fortunately, much work has already been done in this area, notably by the US Department of Energy (DoE) with its Aquatic Species Program that ran for almost two decades, culminating in a final report in 1997.

Algae can be grown on open settling ponds, but this approach is unlikely to provide the best yields. Regrettably, the hardy strains that resist encroachment of viral, fungal and other algae borne in the atmosphere are not the most lipid-rich. Covering ponds with translucent membranes or the use of greenhouses overcomes this drawback, allowing the more productive strains to be grown free of atmospheric contamination. Closed pond systems also enable some control to be exercised over growth factors including the amount of sunlight, water temperature, nutrient mix and concentrations, acidity/alkalinity (pH) of the water and CO₂ concentration.

Photo-bioreactor

Even closed ponds may not be ideal, however, because the growth of a top scum layer tends to block the passage of light to algae lower down in the pond. This has prompted a number of pioneers to abandon ponds altogether, instead adopting fabricated enclosures termed photo-bioreactors (PBRs) that are more three-dimensional. A variety of designs have evolved, all aimed at maximising photosynthesis by slowly circulating the algae, along with nutrients and CO₂, in closed transparent structures that are exposed to light.

US firm Valcent Products, for instance, in a joint venture with Canadian company Global Green Solutions, is growing algae in long rows of suspended moving plastic bags in a patented system called VertiGro. A pilot for the process has been assembled in a large high-density greenhouse near El Paso, Texas. Valcent President and CEO Glen Kertz explains: “By going vertical you can get a lot more surface area to expose cells to sunlight. Our moving system keeps the algae hanging just long enough to pick up the solar energy needed for photosynthesis.”

Kertz told CNN that he could produce up to 100,000 gallons of algal oil a year per acre, compared with 30 gallons/acre from corn or 50 from soybeans. But he admits that algae are no ‘silver bullet’ alternative to oil since it is a “long and winding journey” to cultivate and harness the crop, then extract and refine the algal oil into a usable fuel.

Algae-culturalists using racked glass or polycarbonate PBR systems include Massachusetts-based GreenFuel Technologies Corporation, which aims to utilise waste CO₂ from flue gases, power stations, cement production facilities and other emitters as the source of carbon required by the algae. GreenFuel argues that its solution helps to mitigate CO₂ production at the same time as producing fuel. During 2008 the company raised almost US$14 million of venture capital to use expanding its technology to production scale. A2BE Carbon Capture LLC, which similarly intends using PBRs and waste CO₂, has patented a reactor that is 450 ft long by 50 ft wide and consists of twin transparent plastic algal waterbeds – thus providing parallel redundancy in case a single bed has to be closed down.

Counter-rotating currents induced within the beds ensure maximum exposure of algae to the light as they pass through the phototropic zone. Internal temperature is controlled. For harvesting, a biological agent aggregates the algal cells into larger, more separable entities that can be extracted relatively easily. Internal rollers operating in both directions serve to clean internal surfaces of the waterbed tubes and re-suspend algae.

A2BE co-founders Jim Sears and Mark Allen do not pretend that all the problems have been solved. As Sears, who earlier helped launch another algae-to-oil venture, Solix Biofuels, points out: “You're dealing with adaptive life processes and we need to work with them not against them.”

Despite the difficulties, he anticipates construction of a commercial-scale algae-to-biodiesel plant in 2012. Commentators caution, though, that investors should not be impatient for quick returns.

Meanwhile, Solix Biofuels is working with Colorado State University's Energy Conversion Laboratory on a 20m long fifth-scale PBR that will utilise CO₂ emissions from a brewing facility. Business Development Coordinator Sam Jaffe is clear that the company is engaged in a commercial race to make algae-to-oil technology work on a large scale and at an affordable price. Another PBR exponent, the GreenShift Corporation, headquartered in New York, has produced a pilot-scale reactor with the intention of co-locating it with an ethanol producing facility so that it can utilise CO₂ emissions from that plant.

Enclosed PBRs offer the possibility of achieving highly controlled and optimised growth conditions. But the associated infrastructure, along with that for harvesting the grown algae, has led to systems that, in the view of some experts, have become too complex and expensive. Furthermore, controlling temperature and other parameters, running harvesting machinery, introducing nutrients and capturing waste CO₂ from a fossil fuel burning plant all consume energy. Some companies, such as New Zealand's Aquaflow Bionomics and US company LiveFuels Inc, therefore, remain loyal to the open-pond approach, using alternative techniques to prevent invasion by unwanted competing organisms. LiveFuels has been working with scientists from Sandia National Laboratories in developing a ‘green crude’ product it hopes will be price competitive with fossil crude oil.

Tel Aviv-based Seambiotic Ltd similarly grows a high-yield, oil-rich algal strain in open ponds, using waste CO₂ emissions. In a joint venture with Inventure Capital of Seattle, it is combining its technology with an advanced conversion process developed by Inventure, with the aim of producing biodiesel and ethanol at an intended commercial biofuel plant in Israel. Another youthful company, PetroSun Inc, is pinning its hopes on large saltwater open-pond systems located on the Texas Gulf coast and elsewhere. Raw oil extracted on site would be sent by barge or truck to biodiesel refineries.

Diversified Energy Corporation also believes that closed systems based on PBRs have become over-complex and costly, but has a different answer – a much simplified closed system engineered for low cost. Aiming at agricultural levels of simplicity, DEC has avoided the need for a rigid structure by laying what are essentially transparent plastic tubes in furrows ploughed in the ground. Although this Simgae™ (simple algae) approach requires land, this can be low-grade land that would be unsuitable for food crop cultivation. Infrastructure for CO₂ and nutrient injection and water circulation is based on pumps and piping widely available in agriculture.

Harvesting oils

Harvesting the grown biomass poses an even greater challenge than strain identification and cultivation. At small scale, a producer typically skims wet scum from the area of water it grows on, whether in a pond or a PBR. Then the wet glutinous mass has to be scraped from the skimmer into a receptacle. It is difficult to mechanise and expand this messy and laborious process in a repeatable and reliable way. After it is collected, the damp biomass has to be dried, naturally or in a heated space. Only then can oil extraction take place.

Extracting oils from the dried algae can be as simple as forcing them out with mechanical presses. Alternatively, cell membranes can be broken down with enzymes and chemicals, the oil then being extracted by dissolving it in a solvent – typically hexane, or even water where appropriate algal strains and membrane-eating enzymes are used. Other possible solvents include benzene, ether and CO₂ liquefied under pressure. Alternatively, solvent use can be avoided with methods such as centrifuging and flocculation.

Innovation

Algal fuel development is at an early stage – and wide open to innovation. Although much basic R&D work is still the province of government science and academia, commercial drive is evident in the way the pace of innovation is accelerating. New companies seem to enter the field on a weekly basis, bringing with them radical ideas for enhancing productivity, reducing cost and increasing scale.

One strand of development is aimed at improving yield by selecting appropriate algal strains and in some cases modifying them by selective breeding or genetic manipulation. For example, Florida-based PetroAlgae LLC, recently acquired by investor group PetroTech Holdings Corporation, is employing natural strains of algae developed by Arizona State University and bred selectively over many generations. It cultivates the strains in bioreactors that it plans to scale up for the commercial production of biodiesel and other fuels. It reportedly uses a centrifuging method to extract oil from the harvested and dried algal biomass. Left-over residue constitutes a high-protein meal that can be used as livestock feed.

In an intriguing display of lateral thinking, a California company has developed a novel production method that completely avoids the need for sunlight and photosynthesis. Instead, Solazyme LLC cultivates micro-algae by fermentation in large enclosed tanks, potentially achieving the scale of open pond cultivation without the risk of contamination. The company claims that the process of feeding certain sugars to algae grown in the dark is several times more productive than growing algae in ponds with sunlight. Collecting the biomass is said to be easier than when it is conventionally grown, while reduced capital costs result from less need for equipment and infrastructure.

A possible objection, of course, is that growing the required sugar makes demands on land, energy and water. Moreover, some critics have suggested that avoiding photosynthesis eliminates the chief advantage that algae have over plants, their superior photosynthetic efficiency.

Undaunted, Solazyme has demonstrated Solardiesel fuel produced in this way and oil major Chevron is backing the company financially to develop and test its process at commercial scale. Chevron's Technology Ventures unit is also working with the Nation Renewable Energy Laboratory (NREL) to identify the most suitable algal strains.

Another radical departure that avoids major parts of the conventional process route can be seen in the use of gasification to extract oils. In this approach, dried algae are vaporised with heat. Passing the resultant vapour through a catalyst system provides oil products. Different catalysts encourage the assembly of different organic molecules so that products ranging from crude oil to diesel, kerosene, petrol, etc, can be formed, depending on the catalysts used. This approach avoids the need for the more usual oil extraction, transesterification, refinement/cracking and cleaning processes. The Solena Group, specialists in renewable bio-energy, has leveraged gasification technology developed by NASA. Washington-based Solena uses a plasma gasifier to heat biomass to 5,000 °C, so producing synthetic gas (syngas).

OriginOil Inc of Los Angeles claims to be addressing three primary process issues with patent-pending ‘next generation’ technologies. According to the company, the first challenge is to introduce the CO₂ and nutrients needed for algae growth without agitating the water, since preferred algal strains thrive best in a calm fluid environment. The second is to distribute light evenly within the algae culture, and the third challenge, arising at the oil extraction stage, is to maximise oil yield by cracking the tough walls of as many of the algal cells as possible with the smallest amount of energy.

A process the company calls Quantum Fracturing™ is used to create a slurry of micron-sized nutrition bubbles that are channelled to the algae culture. Increased contact between the micronised nutrients and the algae ensures maximum absorption without fluid disruption or aeration.

Fracturing helps again at the oil extraction stage by encouraging the breakdown of cell walls. In this ‘lysing’ process, water and special catalysts are fractured ultrasonically, with little energy, to help crack the tough membranes. In a pre-cracking stage, algal biomass is subjected to low-wattage microwave bursts to weaken the cell walls. This combination of microwave pre-cracking and ultrasonic cracking avoids the use of energy intensive mechanical methods (which are not always effective), or potentially hazardous solvent chemicals like benzene and hexane.

An even distribution of light is secured through careful design of a Helix BioReactor™, a low-pressure unit in which the culture medium is contained in a rotating vertical shaft around which lights are arranged helically. Growth is optimised by engineering the lighting elements to produce light at specific frequencies. This growth environment allows the algae to replicate exponentially, doubling the colony's biomass in as little as a few hours. OriginOil says that multiple Helix BioReactors can be stacked to form an integrated network of automated, remotely monitored growth units, thus ensuring scalability. For full industrialisation, each reactor group can be connected to a single extraction sub-system to form a complete networked production facility.

Awakening giant?

It is too early to know whether algal biofuels are an awakening giant or whether the hope currently invested in them will be confounded. It seems significant that the US DoE, whose Aquatic Species programme did much to lay the groundwork for algae exploitation but was shelved in the late 1990s for budgetary reasons, recently started work on algae again, through its National Bio-Energy Center (NBC) at NREL. Group Manager of the NBC, Al Darzins, is optimistic about the prospects for algae biofuels, declaring “Wherever there's a lot of sun and a lot of water, you can grow algae. With these organisms, we have the potential to produce 10,000 gallons of oil per acre. In the future, the bulk of the energy on our planet will be produced photosynthetically.”

However, Jim Sears of A2BE cautions that this future might yet be years away, saying: “The journey will be complex, difficult and it's going to take a lot of players.”

The main challenges lie in scaling up the technology for commercial viability. As Jennifer Holmgren, Director of Renewable Energy and Chemicals for Honeywell International's process technology company UOP LLC, comments: “Converting algal oils to fuel is not the fundamental obstacle. The gap is getting the oils in large quantities and demonstrating that they can be manufactured in a cost-effective fashion.”

UOP is working closely with Airbus, International Aero Engines and Jet Blue Airways in a programme aimed at developing biojet fuel for aviation. We will have to wait to see how this develops.